While idle, we need a default chandef to set channel for things,
such as scan. Before support of mac80211 chanctx, mac80211 would
configure a default one on ieee80211_hw::conf::chandef. And we
just queried it whenever we did set channel. However, after support
of mac80211 chanctx, the flow won't work like before.

Besides, we don't now query chandef from ieee80211_hw::conf::chandef
either. So, similar to mac80211 without using chanctx, we configure
the default chandef with ieee80211_channel of index 0 in 2GHz.

Although we have not added the support of mac80211 chanctx here,
this configuration should be compatible before that. So, we commit
this ahead.
Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220809104952.61355-10-pkshih@realtek.com